Subduction Zone Geometry Analysis

Gavin Hayes, NEIC

Introduction

A key step in many earthquake source inversions requires knowledge of the geometry of the fault on which the earthquake occurred. Our knowledge of this surface is often uncertain, however, and as a result fault geometry mislocation can map into significant error in the final temporal and spatial slip patterns of these inversions. Relying solely on an initial hypocenter and CMT mechanism can be risky when establishing rupture characteristics needed for rapid tsunami and ground shaking estimates.
Here we attempt to improve the quality of fast finite-fault inversion results by combining several independent and complementary datasets to more accurately constrain the geometry of the seismic rupture plane of subducting slabs. Unlike previous analyses aimed at defining the general form of the plate interface, we require mechanisms and locations of the seismicity considered in our inversions to be consistent with their occurrence on the plate interface, by limiting events to those with well-constrained depths and with CMT solutions indicative of shallow-dip thrust faulting. We construct probability density functions about each location based on formal assumptions of their depth uncertainty and use these constraints to solve for the 'most likely' fault plane.
In the case of large events (M > ~7.5), these planes can be used directly in new finite fault inversions. For smaller events, this method provides a quick analysis of the tectonic setting of an event and a 'most likely' depth assuming the earthquake occurred on the subduction interface, which can be used as a check against other depth estimates produced at the NEIC.

Data Filtering and Inversion

We filter the historic gCMT catalog to include only shallow thrust mechanisms within 250km of the main shock. We construct the reference profile by calculating the average strike of remaining CMTs deeper than the upper plate, projecting a line 250km downdip from the trench location nearest to the main shock. We remove events greater than 100km distance from the profile in a perpendicular direction. We retain the most accurate hypocenter of each event from a heirarcy of catalogs, first considering the EHB catalog (Engdahl et al., 1998), then the NEIC PDE, and finally the location in the gCMT catalog if neither of these exist. Next we construct probability density functions about the reported depth of the event in the chosen catalog, whose variance reflects the vertical uncertainty of the location. We combine these catalogs with any data available from local active seismic surveys, bathymetry datasets, and sediment thickness catalogs. Finally, we calculate the Most Likely Interface Geometry following Hermite Spline approaches, fitting a line through all available data and the trench location. The non-planar solution varies in dip to match earthquake data in the seismogenic zone, deeper slab earthquakes, and shallow local data where available.

Most Likely Non-Planar Geometry: Strike = 345.8, Dip = 43.1

Result

Figure 1: Basemap of subduction zone showing the area of the trench constrained in this example. Earthquake locations from the gCMT catalog and EHB catalog (gray circles, sized according to magnitude) are shown. Maroon rectangle indicates the area shown in cross section (c); all earthquakes within this area may be used to constrain trench geometry.

Figure 2: Cross-section of subduction zone taken perpendicular to the average strike of gCMTs that match selection criteria and whose equivalent EHB or NEIC locations lie within the maroon box from Figure 1. Gold CMTs are mechanisms from the gCMT catalog plotted at their equivalent EHB catalog location, used to constrain trench strike and dip. Orange CMTs are mechanisms without EHB locations, placed instead at the equivalent event location in the NEIC catalog, and also used to constrain geometry. Light and dark gray circles are events from the EHB catalog in front and behind the plane of the cross-section, respectively, but not used to constrain geometry because either (i) they did not have a corresponding mechanism in the gCMT catalog, or (ii) their mechanism in the gCMT catalog did not match selection criteria. The trench location is marked with a red square. The red dashed line describes the the best-fitting non-planar geometry. The initial location of the reference event is shown by a maroon star and marked with an arrow corresponding to the NEIC epicenter. By clicking on this figure, an expanded cross-section will show the fit between the non-planar geometry and deeper earthquake data (maroon circles), also used to help constrain this geometry. On this section, the dashed blue line represents the local model from Gudmundsson & Sambridge (1998).

Figure 3: Variation in dip of best-fitting fault planes from the gCMT catalog for all events used to constrain trench geometry across the plane of the cross-section. Individual event dips are shown with small dark gray circles, sized with magnitude. Large mechanisms indicate the average dip in 20km bins across the plane of the cross-section. Light gray mechanisms represent a bulk average; dark gray represents a moment-weighted average. The red dashed line represents the dip of the best-fit non-planar geometry.